Charged particle spectrometer and detector therefor

ABSTRACT

A charged particle (e.g. photoelectron) spectrometer is operable in a first mode to produce an energy spectrum relating to the composition of a sample being analysed, and in a second mode to produce a charged particle image of the surface of the sample being analysed. A detector is used to detect charged particles produced in both modes of operation. A method of operation of the spectrometer includes the step of selecting which of said first and second modes to use and the detector being operated accordingly.

The present invention relates to a charged particle spectrometer and toa method of operation of such a spectrometer. In particular, the presentinvention relates to a detector for such a spectrometer.

The bulk of the specification describes the application of the inventionin a photoelectron spectrometer, but other charged particle instrumentswould also be suitable. For example, a hemispherical-only analysersystem where the input lens system is operated in such a way as toproject a line image from the specimen that is then dispersed in theorthogonal direction to generate a 2d image, with one axis beingpositioned along a line on the sample and the other showingphotoelectron energy. Alternatively, the input lens system could beoperated to project an angular distribution from the sample as in forexample a Thermo VG Scientific Theta Probe.

Also, for example, the invention could be applied to spectrometers usingAuger electrons or scattered ions as the analysed charged particles.

A current photoelectron spectrometer produced by the applicant is shownschematically in FIG. 1. The instrument consists of a magnetic lens 2above which is located the sample 4 to be analysed. In use, the sample 4is bombarded by X-rays from an X-ray source 6 and the photoelectronsproduced are passed through a charge neutraliser 8 and an electrostaticlens system 10 so as to be focused at an entry 12 to an energy analysingsection 14.

The instrument has two modes of operation: a spectrum mode for analysingthe composition of the surface of the sample 4; and an imaging mode forproducing a magnified energy selected photoelectron image of the surfaceof the sample 4. In the spectrum mode, the photoelectrons pass around ahemispherical analyser 16 and are received by a pair of detectors 18,20, which are typically each a set of channeltrons. The two sets ofchanneltrons enable the instrument to produce an energy spectrumrelating to the composition of the surface of the sample 4 from whichthat composition can be analysed.

In imaging mode, the photoelectrons pass through spherical mirroranalyser portion 22 of the energy analysing section 14 and are receivedby a different detector 24 which is typically a micro channel plate(MCP) detector. Photoelectrons received by the micro channel plate areused to produce further secondary electrons which are then projected onto a phosphorescent screen. The phosphorescent screen can then be viewedby a CCD camera from which an energy analysed photoelectron image of thesurface of the sample 4 can be produced. Said image could represent thedistribution of a particular element or chemical state of the element.

This instrument has the disadvantage that two types of detectors arerequired as explained above, one for each mode of operation. The presentinvention aims to reduce or overcome some or all of the disadvantagesassociated with prior art instruments.

Accordingly, in a first aspect, the present invention provides a chargedparticle spectrometer which is operable in a first mode to produce anenergy spectrum relating to the composition of a sample being analysed,and in a second mode to produce a charged particle image of the surfaceof the sample being analysed, wherein the spectrometer includes adetector which is used to detect charged particles produced in bothmodes of operation.

The charged particles could be photoelectrons, auger electrons or othersecondary electrons from the specimen or even ions if the spectrometerwas to be used for ion scattering spectroscopy.

In this way, the present invention reduces the complexity of thedetector system of the prior art instrument. Also the detector canreceive charged particles, e.g. photoelectrons, over a larger physicalarea than is the case with the prior art since in the prior art the twotypes of detector can not be located in the same physical location andso each detector in use is only covering a part of the detection area.

Preferably the charged particle spectrometer is a photoelectronspectrometer, wherein the charged particle image is a photoelectronimage, and wherein the charged particles are photoelectrons.

Preferably the detector includes plate means (such as a micro channelplate) on to which in use primary electrons are directed in both modesof operation and which emits a plurality of secondary electrons for eachprimary electron received. Preferably the detector also includes firstdelay line means for using the plurality of secondary electrons toproduce a pair of electrical pulses in a delay line from which a signalprocessing means can calculate the location of the primary electron onthe plate means in a first direction. More preferably, the detector alsoincludes second delay line means for using the plurality of secondaryelectrons to produce a pair of electrical pulses in a second delay linefrom which the signal processing means can calculate the location of theprimary electron on the plate means in a second direction.

Effectively, this type of detector partly replaces the phosphorescentscreen and CCD detector as described in the prior art. This enables thelocation of each primary electron on the plate means to be determinedmore accurately.

Preferably the first and second directions are orthogonal e.g.effectively define an X and Y axis on the plate means.

In some embodiments the spectrometer includes second signal processingmeans (which it may be separate from, or part of the signal processingmeans mentioned above) for processing the signals received from one orboth of the delay lines in order to reduce or eliminate any unwantedsignals, such as noise caused by imperfections in the construction ofthe detector and/or electronic cross talk between the delay lines.

Preferably the spectrometer includes control means for controlling itsoperation and enabling a user to select which of the two modes isoperating. Preferably the control means also controls the signalprocessing means such that when the spectrometer is operating inspectrum mode, the signal processing means utilises signals from onlyone of the delay line means.

Additionally or alternatively, the control means may also control thesignal processing means so that when the spectrometer is operating inimage mode the signal processing means utilises signals from both thefirst and second delay line means and may also include furtherprocessing means for increasing the accuracy of the time measurements ofthe electrical pulses, preferably by stretching the time between eachone of a pair of pulses so that the time difference may be moreaccurately measured.

In a further aspect, the present invention provides a detector for acharged particle spectrometer, the detector including any or all of thefeatures described above.

In a further aspect, the present invention provides a method ofoperating the charged particle spectrometer as described above whereinthe method includes the step of selecting which of the two modes to useand the detector being operated accordingly.

An embodiment of the present invention will now be described withreference to the accompanying drawings in which:

FIG. 1 is a schematic diagram of a prior art photoelectron spectrometer.

FIG. 2 is a schematic diagram of a photoelectron spectrometer accordingto the present invention.

FIG. 3 is a schematic diagram showing part of a detector according to anembodiment of the present invention.

FIG. 4 is a flow chart showing the operation of a spectrometer accordingto an embodiment of the present invention.

FIG. 5 is a schematic diagram showing the operation of a detectoraccording to an embodiment of the present invention in spectroscopy orspectrum mode.

FIG. 6 is a schematic diagram showing part of a detector according to anembodiment of the present invention and its operation in imaging mode.

FIG. 7 is a schematic diagram showing a further delay line anodeassembly.

FIG. 2 shows a schematic diagram of an XPS (X-ray photoelectronspectrometer) which in its basic operation is fairly similar to theinstrument shown in FIG. 1. Identical reference numerals have been usedfor those parts of the instrument which are the same. The maindifferences lie in the detector used.

In FIG. 2, the spectrometer includes a single detector unit 30 which isusable in both modes of operation of the spectrometer—spectrum mode andimaging mode. In some embodiments, the detector plate 30 is a microchannel plate (MCP) and in some other embodiments it may include aplurality of micro channel plates, such as three or more plates.

Arranged adjacent to the detector plate 30 is a pair of delay lines 32,34, although more or fewer delay lines may be used. The detector plate30 and the delay lines 32, 34 together make up the detector of thisinstrument and this detector is usable for both imaging andspectroscopy, unlike the prior art instrument described above.

FIG. 3 shows in schematic form the operation of part of the detector. Inuse, in either mode of operation, primary electrons from the instrumentwill strike the micro channel plate (MCP) 40. In FIG. 3, a singleelectron 42 is schematically shown striking the micro channel plate 40.The operation of the detector plate, such as an MCP, is to amplify asingle electron by a large factor (e.g. 10⁷) to produce a “shower” 44 ofsecondary electrons. A delay line 46 is arranged in a suitable positionso that the shower 44 of electrons may fall on it or strike it. As shownin this embodiment, the delay line 46 is arranged such that it coversall or substantially all of the area of the detector plate and alsopreferably such that the line is laid out in a serpentine fashionwhereby the elongate parts of the line are parallel or substantiallyparallel. However, other arrangements of the delay line are possiblesuch as that produced by winding the delay line around a former toproduce a helically wound delay line.

In this way, the elongate parts of the delay line 46 may be arranged tolie perpendicular to a chosen axis of the detector plate. In thisexample, the delay line 46 lies perpendicular to what is shown as the“X” axis and so the delay line is called the “X” delay line.

The function of the delay line 46 is such that the shower of secondaryelectrons striking it produces a pair of pulses 48, 50 which propagatetin respectively different directions along the delay line i.e. onepulse 50 propagates towards a first end 52 and the second pulse 48propagates towards a second end 54. The ends of the delay line may beconnected to signal processing means which receives the pulses 48, 50and calculates the time difference between their times of receipt, shownschematically in FIG. 3. This time difference enables the point ororigin 56 of the shower 44 on the delay line 46 to be calculated, or atleast its coordinate in the “X” direction. This correlates to theposition at which the primary electron 42 struck the detector plate andso the position of that electron in the “X” direction can be determined.

In some of the embodiments, the detector may include a second delay linewhich functions as described above but is laid out in a different way.Preferably the second delay line is laid out so that its elongate partslie perpendicular to a different axis to the “X” axis and morepreferably that different axis is orthogonal to the “X” axis e.g. the“Y” axis shown in FIG. 3. In this way, the position of the primaryelectron 42 may be determined with respect to both axes i.e. its preciselocation on the detector plate can be known if necessary depending onthe mode of operation of the spectrometer. A second delay line 58 isshown in FIG. 3, which lies perpendicular to what is shown as the “Y”axis and so the second delay line is called the “Y” delay line.

Other arrangements of the delay lines are possible, so that the elongateparts of the delay line may be arranged to lie parallel to a chosen axisof the detector plate. For example the elongate parts of an “X” delayline may lie parallel to an “X” axis, and those of a “Y” delay line maylie parallel to a “Y” axis, wherein the “X” delay line enables thecoordinate in the “X” direction of a shower of secondary electrons to becalculated, and wherein the “Y” delay line enables the coordinate of theshower in the “Y” direction to be calculated.

The spectrometer of one aspect of the present invention may be operablein either one of two different modes as mentioned above—a spectrum modeand an imaging mode. FIG. 4 is a flow chart showing an overview of theoperation in both modes. As can be seen, in spectrum mode only readingsin only one dimension are required at the detector and so only a portionof the detector may be used. In the detector embodiment utilising a pairof delay lines as described above, this means that the signal processingmeans may operate on only signals received from one of the delay linese.g. the “X” delay line 46 as shown in FIG. 3. This is also shown inmore detail in FIG. 5.

FIG. 4 also shows the operation of the spectrometer in the imaging modein which data from two dimensions on the detector is desired. In thedetector embodiment described above utilising a pair of delay lines,this means that the outputs of both delay lines will be utilised by thesignal processing means as previously described in order to determinethe position of the primary electrons on the detector.

FIG. 5 shows schematically how a single delay line 60 is utilised todetermine a measurement of the energy of photoelectrons falling on thedetector plate. By the nature of the operation of the spectrometer, thefurther along the “X” axis at which an electron strikes the detectorplate, the greater its energy. The delay line 60 is used as previouslyexplained in order to determine the position of electron strike in this“X” direction. In this mode, one or more “time stretchers” may be usedin order to enhance the time resolution available for calculating thetime difference between pulses in a pair of pulses on each delay line.

In the 1 dimensional single delay line mode because the stretchers maynot be needed since there may be no need to enhance the time resolutionin this mode. In this mode it is usually more important to maximise thecount rate and time stretchers reduce the maximum rate at which eventscan be processed because they extend the required acquisition time foreach event. However in an application where enhanced resolution wasrequired then it would be desirable to use time stretchers.

FIG. 6 shows in schematic form the operation of part of the detector inimaging mode. A “shower” 74 of secondary electrons is schematicallyshown striking the “X” delay line 76, and the “Y” delay line 78. Asshown in FIG. 6, the elongate parts of the delay line 76 lieperpendicular to the “X” axis and the elongate parts of the delay line78 lie perpendicular to the “Y” axis. The shower of secondary electrons74 produces a pair of pulses in each of the delay lines, which propagateto different ends thereof. The ends of the delay lines may be connectedto signal processing means which receives the pulses and calculates thepoint or origin 70 of the shower on the delay lines. The pulses in the“X” delay line 76 enable the signal processing means to determine thecoordinate of the shower 74 on that delay line in the “X” direction, andthe pulses in the “Y” delay line 78 enable the signal processing meansto determine the coordinate of the shower on the “Y” delay line in the“Y” direction. In this way, a photoelectron image 72 may be produced,which may be a magnified photoelectron image of the surface of thesample in the spectrometer.

A detailed embodiment of the detector electronics will now be describedin order to illustrate the operation of both modes:

The position of an electron impact on the detector is determined usingan electronic system.

When an electron hits the front of the detector micro-channel plate(MCP) it causes a current pulse from the MCP power supply, as anavalanche of secondary electrons is created. The current pulse may bedetected as a voltage pulse across a resistor. Preferably, afteramplification, if the pulse exceeds a predefined threshold, an ECL(emitter coupled logic) “start” pulse is generated e.g. using a constantfraction discriminator circuit (CFD). The CFD may be used rather than asimple threshold detector so that the timing of the ECL signal isrelated to the peak of the voltage pulse, and is independent of theamplitude of the pulse. Other types of logic interface may also be used.

The logic interface is a description of the type of signal processingelectronic components. ECL is one type, other types are, for example,Low Voltage Differential Signalling (LVDS) or Low Voltage Positive ECL(LVPECL). The CFD function may be performed by any of the above “logicinterface” standards.

The electron cloud leaving the back of the MCP hits the detectorwire(s), and respective current pulses propagate to both ends of eachdetector wire. They are detected e.g. as voltage pulses acrossresistors, may be amplified and preferably ECL “stop” pulses aregenerated using CFDs as before. The position of the electron impact onthe detector can be determined by timing between the start pulse and thestop pulses, using the position measurement electronics. The differencebetween the two times for each wire indicates the distance of the impactposition from the centre of the wire. The sum of the two times for eachwire should be constant, and can be used to detect and rejectoverlapping impacts.

The position measurement electronics uses e.g. multi-channeltime-to-digital converter (TDC) integrated circuits to measure the startto stop periods. The stop pulses are enabled into the circuitry by thearrival of a start pulse, to prevent spurious stop pulses causinginvalid measurements. A timeout period may be used to reset thecircuitry if the stop pulses are not received within the maximum startto stop duration. In this example, valid ECL signals are converted topositive ECL (PECL) and are passed to the TDC inputs. Typically, theTDCs are capable of timing start to stop periods to a 500 ps resolution.

As described before, the electronics has two modes of operation: asingle dimensional mode and a two dimensional mode. In the singledimensional mode the stop pulses from only one of the detector windingsare used. In this mode the typically 500 ps resolution of the TDC issufficient, but a high TDC throughput is desired. This is achieved bymultiplexing the start and stop pulses to each of a plurality e.g. four,TDCs in turn. While one device is timing an event, the other device(s)are at different stages of outputting their data to a storage device,e.g. FIFO (“first in first out”), under the control of a hardware statemachine. The times are then read from the FIFO into a digital signalprocessor (DSP) for processing.

In the two dimensional mode the signals from both detector windings areused. In this mode an improved time resolution of typically 50 ps isachieved using a time stretching circuit. In one example, a capacitor ischarged to a set voltage, prior to operation of the time stretchercircuit. During the start to stop period the capacitor is negativelycharged using a fixed constant current, such that the capacitor voltagecrosses a threshold just below the initial voltage and continues toincrease negatively until the end of the start to stop period. At theend of this period the capacitor is charged positively at a slower rateusing a lower constant current, back to the initial voltage. As thecapacitor voltage crosses the threshold voltage, a high-speed comparatorproduces a stop signal, which is passed to the TDC. The amount the timeis stretched is determined by the ratio of the discharging current tothe charging current.

A lower throughput is required in two-dimensional mode, so the timestretching and the need to read four values out of the TDC rather thantwo, does not cause a throughput problem. It is also possible to use asingle TDC in this mode to eliminate small timing offset differences,caused by manufacturing process differences between TDCs, which mayotherwise be experienced.

It is possible that images captured using the delay line detector maycontain distortions which appear as faint horizontal and verticalstripes. These are thought to be caused by imperfections in theconstruction of the detector and/or electronic cross talk between thefour stop signals. The invention may use a calibration method whichreduces these artefacts.

It is assumed that the stripes are caused by the detector system“moving” electron events slightly from their true positions, dependingon their positions in the image and that the error in the horizontal (X)position is independent of the vertical (Y) position and vice versa.Where the image is too bright, the electron events are moved away fromeach other and where the image is not bright enough, the electron eventsare moved closer together. Each electron event's position can becorrected independently for X and Y. The calibration consists of twotables containing a position adjustment for each X and Y position.

This procedure causes a slight loss of spatial resolution, but becauseadjustments are small the loss of resolution is small compared with theinstrument resolution. There is no effect on image intensity, since theoverall number of electron events remains the same.

The calibration tables are generated using a reference image obtained byuniformly illuminating the detector with charged particles.

The procedure for generating the correction table for X positions of animage is described. The procedure for Y is identical. As an example, theimage is assumed to be 500 points by 500. The reference image consistsof a list of X and Y co-ordinates in the range 0-499. The total numberof electron events should be as large as is practical, typically severalmillion.

-   1. The total number of electron events at each X position    (regardless of Y position) is calculated, giving a array of 500    intensities. Each element represents the total intensity of a    vertical line of the image.-   2. The list of intensities is normalised by dividing each intensity    by the average of all intensities and subtracting 1.0. This gives a    list of positive and negative values close to zero and represents    the error in intensity at each position.-   3. The calibration table of position adjustments is derived as    follows.    -   The position adjustment for the first point (co-ordinate        value 0) is set to half the intensity error for the first point.    -   For all other points except the last, starting with the 2nd        point and working up, the position adjustment is set to the        position adjustment of the previous point added to the average        intensity error of the previous and current points.    -   For the last point (co-ordinate value 499), the position        adjustment is set to the position adjustment of the previous        point (co-ordinate value 498) added to half the intensity error        of the last point.        The co-ordinates for each electron event are adjusted by adding        the appropriate X position adjustment to the X co-ordinate and        the appropriate Y position adjustment to the Y co-ordinate. This        results in co-ordinates which are real numbers, not integers and        some co-ordinates may be less than 0.0 or greater than 499.0.

Often, it is necessary to convert the co-ordinates to integers. Becausethe calibration corrections are typically less than 1.0, simplytruncating or rounding the co-ordinates to integers would not giveacceptable results. In order to convert the co-ordinates to integers analgorithm is used which rounds up or down at random, with theprobability of rounding up depending on the magnitude of the fractionalpart. This is done by adding a random number between 0.0 and 0.9999999to each co-ordinate and then truncating to an integer.

FIG. 7 shows a diagram of a delay line anode assembly that includes someadditional electrodes. These are flat rectangular collector plates (80)for the electron clouds emitted by the MCPs that can be used instead of(or as well as) one of the delay lines to detect the position of theelectron events along one of the directions.

The plates (80) are mounted behind the delay line wires (not shown onthis diagram—just the semicircular delay line guides (82) are shown) andthe charge emitted by the MCP can be preferentially collected by theirby changing the relative potentials on the delay line wires and thediscrete anodes. A second array of plates could be added so that eachdelay line had a corresponding array of plates.

The plates (80) could each be connected to a separate amplifierdiscriminator counter channels. They have the advantage of being able torecord a higher overall count rate from the detector for certain highcount rate applications but at reduced positional resolution (theresolution is determined by the size of each plate). The delay linedetector system, (timing the pulses at the ends of the line) may belimited to a few million events per second. Some signal sources for thespectrometer can produce signal levels of e.g. 10 times this so in thiscase this third mode of operation using separate discrete anodes may beappropriate.

The above embodiments are intended to be an example of the presentinvention and variants and modifications of those embodiments, such aswould be readily apparent to the skilled person, are envisaged and maybe made without departing from the scope of the present invention.

1. A charged particle spectrometer which is operable in a first mode toproduce an energy spectrum relating to the composition of a sample beinganalysed, and in a second mode to produce a charged particle image ofthe surface of the sample being analysed, wherein the spectrometerincludes a detector which is used to detect charged particles producedin both modes of operation.
 2. A charged particle spectrometer accordingto claim 1 which is a photoelectron spectrometer, wherein the chargedparticle image is a photoelectron image, and wherein the chargedparticles are photoelectrons.
 3. A charged particle spectrometeraccording to claim 1 wherein the detector includes a plate means, on towhich, in use, primary electrons are directed in both modes ofoperation, and which emits a plurality of secondary electrons for eachprimary electron received.
 4. A charged particle spectrometer accordingto claim 3 wherein the plate means is a micro channel plate.
 5. Acharged particle spectrometer according to claim 3 wherein the detectoralso includes a first delay line means for using the plurality ofsecondary electrons to produce a pair of electrical pulses in a firstdelay line from which a signal processing means can calculate thelocation of the primary electron on the plate means in a firstdirection.
 6. A charged particle spectrometer according to claim 5wherein the detector also includes a second delay line means for usingthe plurality of secondary electrons to produce a pair of electricalpulses in a second delay line from which the signal processing means cancalculate the location of the primary electron on the plate means in asecond direction.
 7. A charged particle spectrometer according to claim6 wherein the first and second directions are orthogonal.
 8. A chargedparticle spectrometer according to claim 5 wherein second signalprocessing means processes the signals received from one or both of thedelay lines to reduce or eliminate any unwanted signals.
 9. A chargedparticle spectrometer according to claim 5 including a control means forcontrolling its operation and enabling a user to select which of the twomodes is operating.
 10. A charged particle spectrometer according toclaim 9 wherein the control means also controls the signal processingmeans such that when the spectrometer is operating in said first mode,the signal processing means utilises signals from only one of the delayline means.
 11. A charged particle spectrometer according to claim 9wherein the control means also controls the signal processing means sothat when the spectrometer is operating in said second mode the signalprocessing means utilises signals from both the first and second delayline means.
 12. A charged particle spectrometer according to claim 9wherein the control means includes further processing means forincreasing the accuracy of time measurements of the electrical pulses.13. A charged particle spectrometer according to claim 12 wherein thefurther processing means increases said accuracy by stretching the timebetween each one of a pair of pulses so that the time difference may bemore accurately measured.
 14. A detector for a charged particlespectrometer, which spectrometer is operable in a first mode to producean energy spectrum relating to the composition of a sample beinganalysed, and in a second mode to produce a charged particle image ofthe surface of the sample being analysed, wherein the detector is usableto detect charged particles produced in both modes of operation.
 15. Adetector according to claim 14 wherein the charged particle spectrometeris a photoelectron spectrometer, the charged particle image is aphotoelectron image, and the charged particles produced in both modes ofoperation are photoelectrons.
 16. A detector according to claim 14including a plate means, on to which, in use, primary electrons aredirected in both modes of operation, and which emits a plurality ofsecondary electrons for each primary electron received.
 17. A detectoraccording to claim 16 wherein the plate means is a micro channel plate.18. A detector according to claim 16 also including a first delay linemeans for using the plurality of secondary electrons to produce a pairof electrical pulses in a first delay line from which a signalprocessing means can calculate the location of the primary electron onthe plate means in a first direction.
 19. A detector according to claim18 also including a second delay line means for using the plurality ofsecondary electrons to produce a pair of electrical pulses in a seconddelay line from which the signal processing means can calculate thelocation of the primary electron on the plate means in a seconddirection.
 20. A detector according to claim 19 wherein the first andsecond directions are orthogonal.
 21. A detector according to claim 18wherein the signal processing means processes the signals received fromone or both of the delay lines to reduce or eliminate any unwantedsignals.
 22. A detector according to claim 19 wherein the signalprocessing means utilises signals from only one of the delay line meanswhen the spectrometer is operating in said first mode.
 23. A detectoraccording to claim 22 wherein the signal processing means utilisessignals from both the first and second delay line means when thespectrometer is operating in said second mode.
 24. A detector accordingto claim 18 wherein further processing means increase the accuracy oftime measurements of the electrical pulses.
 25. A detector according toclaim 24 wherein the further processing means increases said accuracy bystretching the time between each one of a pair of pulses so that thetime difference may be more accurately measured.
 26. A method ofoperation of a charged particle spectrometer according to claim 1wherein the method includes the step of selecting which of said firstand second modes to use and the detector being operated accordingly.